Epidemic typhus, also known as Gaol fever, Jail fever, or Famine fever, is caused by the Gram-negative bacterium Rickettsia prowazekii. This louse-borne disease has been responsible for devastating widespread epidemics throughout history, especially during and after the wars, famine, and poor socio-economic conditions, and is the only known rickettsiosis that can recur after a long period of latency (Brill-Zinsser disease). R. prowazekii is classified as a CDC/NIAID Category B biological warfare pathogen, the entire genome for which was the first to be sequenced among all known Rickettsia species. Although tremendous strides have recently been made in the genetic manipulation of R. prowazekii, the knowledge of molecular aspects of interactions with the vascular endothelium, the preferred cell type infected during human infections, still remains in its infancy. Endothelial cells, key immunoreactive cells involved in host defense and inflammation, are also intimately involved in the manifestations of rickettsial infections. On the basis of interactions of host endothelial cells with R. prowazekii and R. typhi (the etiologic agent of endemic typhus), we have identified activation of nuclear factor kappa B (NF-?B) and stress-activated p38 protein kinase as critically important regulatory signaling mechanisms that contribute to host cell activation and responses to infection. Specific Aim 1 of this exploratory grant application is designed to define the intensity and kinetics of the activation of NF-?B and MAP kinase signaling pathways after R. prowazekii infection of vascular endothelial cells in vitro and test the hypothesis that potential differences in intracellular signaling mechanisms determine the intensity of host cell activation in response to virulent Breinl versus attenuated Madrid E versus virulent revertant Evir strains of R. prowazekii. Aim 2 will focus on establishing and characterizing a mouse model of R. prowazekii infection that closely mimics the major pathological features, i.e. disseminated endothelial infection and vascular inflammation, of epidemic typhus disease in humans. Taken together, these studies will address critical gaps in our current understanding of in vitro and in vivo interactions between vascular endothelium and R. prowazekii and mechanisms underlying 'endothelial activation'during typhus rickettsioses. The long-term objective of this project is to establish the foundation for comprehensive understanding of epidemic typhus pathogenesis by identifying specialized vascular cell signaling pathways activated in vitro and subsequent detailed analysis of their involvement in determination of innate and adaptive immune responses with an aim to develop unique chemotherapeutic strategies focused at targeted intervention. In addition, detailed characterization of a small animal model of infection akin to disease in humans will allow us to define unique features of virulence factors and pathogenesis of epidemic typhus, to expand our understanding of in vivo immune responses, and to test the efficacy of novel antibiotics and vaccine candidates. PUBLIC HEALTH RELEVANCE: Typhus epidemics due to louse-borne Rickettsia prowazekii have caused more deaths than all the wars combined and recent epidemiological evidence documents the reemergence of epidemic typhus in different geographic locations of the world. Since vascular dysfunction and damage are the major pathologic sequelae responsible for complications of human rickettsial diseases, obtaining a definition of signaling interactions between host endothelial cells and R. prowazekii strains of varying virulence and detailed characterization of a mouse model of disseminated infection of the vasculature represent first major steps in advancing our understanding of pathogenesis of epidemic typhus, which will ultimately lead to the development of novel therapeutic and immunologic strategies to combat this debilitating rickettsial disease.